Research progress on utilization and separation of depolymerized products of lignite

Lignite,as a typical low rank coal,has low utilization efficiency as primary energy source. However,lignite contains abundant natural structural units and has high chemical reactivity,making it possible to utilize lignite as non-energy resource. Therefore,exploring some value-added approaches of non-energy resource use of lignite becomes the realistic demand and an inevitable choice for the utilization of lignite and other low rank coals. Lignite depolymerization is a potential way to realize the high value utilization of lignite. However,the depolymerized products are rather complex mixtures,and how to realize the effective utilization of depolymerized mixtures is a bottleneck problem that limits the utilization of lignite depolymerization. At present,the reports on lignite depolymerization are mostly focused on the establishment and optimization of lignite depolymerization methods,while there are few reports on how to use the obtained depolymerized products,and there is still a lack of special review on the utilization of lignite depolymerized products and their separation. This review pays special attention on the research progress concerning the utilization of the lignite depolymerized products and the separation approaches,including some different depolymerization methods and composition characteristics of depolymerized products,the reported depolymerization product utilization approaches,and the separation process of depolymerized products,etc.At present,the utilization of lignite depolymerized products mainly includes two aspects. One is to speculate the original structure of coal according to the depolymerized products,which has been widely reported. The second is the functional utilization of depolymerized products due to their characteristics,including direct utilization without separation and the utilization of monomers after separation,which is also being gradually explored. In terms of the exploration of separation methods of depolymerized products,in addition to the common centrifugation,filtration,extraction and column chromatography,the low eutectic solvent method,anti-solvent method and metal ion coordination separation methods are also gradually used for the separation of such complex systems. On the basis of the above literature research,the existing problems and future development trends are analyzed.